This invention relates to a cage for use in a rolling bearing.
Heretofore, as a cage for use in a rolling bearing under going large load, a machined cage made of high strength brass excellent in the mechanical strength has been used. While the cage is excellent in the slidability and the wear resistance due to self-lubricity, since the material cost is high, the fabrications cost is high, and the yield is low, it is used only for special applications.
On the contrary, a pressed cage formed by fabricating a cold rolled steel sheet typically represented by SPCC or a hot rolled steel sheet typically represented by SPDH by pressing into a predetermined shape is advantageous in view of the cost compared with the machined cage made of high strength brass, but it is poor in the slidability and the wear resistance. Therefore, it has been conducted to improve the slidability and the wear resistance of the pressed cage by applying nitriding thereby forming a hard nitride layer on the surface of the cage.
Generally, when nitriding is applied to an iron and steel member comprising a low carbon steel such as SPCC or SPDH (iron and steel material with carbon content of 0.25% by weight or less), nitrogen and iron are reacted on the surface of the iron and steel member, by which nitrogen atoms diffuse into the iron and steel member and a layer comprising a nitrogen compound (nitride layer) is formed on the surface. The nitride layer forms a phase at higher nitrogen concentration toward the surface of the member. That is, the phase of the nitride layer changes from the vicinity at the boundary with the base material at the innermost site toward the outermost surface in the order of xcex3xe2x80x2 phase (Fe4N), xcex5 phase (Fe2-3N), and "xgr" phase(Fe2N).
In the nitriding conducted in existent pressed cage (xe2x80x9cTufftride methodxe2x80x9d and gas softening nitriding), since nitriding is applied at a high temperature of 550-600xc2x0 C., the xcex5 phase and the "xgr" phase at the surface tend to form a porous structure. When the distribution for the hardness in the surface layer portion of the nitride layer obtained by the gas softening nitriding method by a nano-indentation method accurately at an interval of several micro orders, while the hardness is Hv 600 or more at the depth of about 10 xcexcm but it is sometimes less than Hv 500 at the outermost surface as shown at (b) in the graph of FIG. 3. In the measurement for the hardness by an existent micro-vickers tester, the distribution for the hardness on the surface layer of the nitride layer can not be measured at an interval of several xcexcm.
Further, in the existent method describe above, the nitride layer is formed at a large thickness by making the processing time longer upon nitriding or increasing the nitrogen potential in the treating atmosphere while considering the decrease in the thickness of the nitride layer by wear during use of the cage. In accordance therewith, the nitrogen concentration in the nitride layer becomes higher and most of the nitride layer sometimes forms the "xgr" phase. The "xgr" phase is poor in the toughness and tends to suffer from sharing fracture easily by sliding friction with rolling elements and tends to drop easily from the surface of the cage. As a result, it is difficult to obtain the effect of improving the slidability and the wear resistance.
On the other hand, Japanese Published Unexamined Patent Publication Hei 10-2336 describes that fluoridation of replacing the oxide on the surface of the cage with a metal fluoride film before nitriding forms a uniform and dense nitride layer comprising nitrides with an average grain size of 1 xcexcm or less on the surface of the cage. However, since the wear resistance of the cage greatly depends on the type and the hardness of the nitride layer, the slidability and the wear resistance are sometimes insufficient by merely making the nitride grain size finer on the outermost surface like the cage described in the above noted publication.
Further, Japanese Published Unexamined Patent Publication 2001-90734 proposes formation of a dense nitride layer of 3-20 xcexcm thickness and a nitride layer of a porous structure of 2-25 xcexcm thickness on the surface of a pressed cage by nitriding. According to this proposal, both the surface hardness and the lubricant retaining performance of the pressed cage are made favorable and improvement is expected for the wear resistance. However, even by the proposal, no sufficient wear resistance can sometimes be obtained in a severe lubrication circumstance in which a lubricant film is less formed.
Further, it is also necessary to prevent degradation of the mechanical strength of the cage by the nitriding.
The present invention has been accomplished taking notice on the subject in the prior art as described above, and it is an object thereof to provide a cage more excellent in the slidability and the wear resistance than usual in a cage in which a nitride layer is formed on the surface by using an iron and steel material with carbon content of 0.25% by weight or less, fabricating the same into a predetermined shape and applying nitriding, and to increase the mechanical strength of the cage.
For solving the subject described above, the present invention provides a cage in which a nitride layer is formed on a surface by using an iron and steel material with a carbon content of 0.25% by weight or less and fabricating the same into a predetermined shape and then conducting nitriding, having a nitride layer with a hardness of 650 or more in vickers hardness (Hv) at a thickness of 3 or more and 15 xcexcm or less.
By the way, even when it is intended to measure the distribution of the hardness in the direction of the depth of a thin layer such as a nitride layer in the measurement for the hardness by an existent micro-vickers tester at an interval of several micron meters, no accurate measurement is possible since reading error is large or indentation diameter increases due to excessive load, but accurate measurement is possible by the nano-indentation method. Accordingly, the hardness of the nitride layer in the cage of the present invention should be measured by a nano-indentation method. Further, when it is measured by the nano-indentation method, measurement is conducted in a state of where the indentation diameter is at least 5 to 50% for the thickness of the nitride layer.
The cage of the present invention is obtained by applying the nitriding to the cage formed of an iron and steel material such that formation of the "xgr" phase is suppressed and a nitride layer comprising xcex3xe2x80x2 phase, xcex5 phase or(xcex3xe2x80x2+xcex5)phase is formed. As the method, it can be mentioned that the reaction rate of nitriding is retarded and the temperature for nitriding is preferably 400 to 550xc2x0 C. and, more preferably, 400 to 540xc2x0 C.
Further, when nitriding is applied at relatively high temperature after forming the cage by pressing from a steel sheet formed of a low carbon steel, since work strains during pressing are relieved during nitriding, the cage tends to form deformation. However, the heat treatment deformation caused to the cage can be suppressed by applying the nitriding in the temperature range described above that is a temperature range in which recrystallization dose not occur in the low carbon steel.
When the temperature for the nitriding is 560xc2x0 C. or higher or the treatment time is longer, since the thickness of the nitride layer exceeds 15 xcexcm in which the "xgr" phase is formed to the outermost surface of the nitride layer making it porous and deteriorating the hardness and the toughness of the nitride layer, no good slidability and wear resistance can be obtained. On the contrary, when the thickness of the nitride layer is less than 3 xcexcm, the thickness of the nitride layer tends to become not uniform possibly forming a portion where the nitride layer is not formed, good slidability and wear resistance can not be obtained reliably.
Good slidability and wear resistance can be obtained reliably by controlling the thickness of the nitride layer to 3 xcexcm or more and 15 xcexcm or less. The thickness of the nitride layer is, preferably, 5 xcexcm or more and 13 xcexcm or less and, more preferably, 8 xcexcm or more and 12 xcexcm or less.
That is, the cage of the present invention comprises an iron and steel material with the carbon content of 0.25% by weight or less in which a nitride layer comprising not "xgr" phase but xcex3xe2x80x2 phase, xcex5 phase or (xcex3xe2x80x2+xcex5) phase is formed on the surface. Thus, as shown in by (a) in the graph of FIG. 3, in the cage of the present invention, the hardness of the nitride layer is Hv 650 or more even at a depth of about 1 xcexcm from the surface and it is formed to a substantially uniform hardness along the direction of the thickness of the nitride layer.
The method of forming a nitride layer comprising xcex3xe2x80x2 phase, xcex5 phase (xcex3xe2x80x2+xcex5) phase to the surface of a cage formed of an iron and steel material with the carbon contend of 0.25% by weight or less can include, for example, (1) a method of keeping in a mixed gas of a nitrogen gas and an ammonia gas at 450-540xc2x0 C. for 2-3 hours (gas soft nitriding method at low temperature), (2) a method of applying fluoridation by using a fluorine gas such as NF3 (nitrogen trifluoride) at about 200-400xc2x0 C. and then keeping the same in an NH3 gas at 400-540xc2x0 C. for 1-3 hours(xe2x80x9cNv super nitridingxe2x80x9d, commercial name of Air Water Inc.), (3) a method of dipping in a special salt bath and keeping at a low temperature of 450-530xc2x0 C. for 1-3 hours by xe2x80x9cTufftride methodxe2x80x9d for 1-3 hours (xe2x80x9cPalsonite treatmentxe2x80x9d, registered trademark of (Nippon Parkarizing Co.) and (4) a method of applying glow discharge at a temperature of about 500-550xc2x0 C. for about 10 hours in a mixed gas atmosphere of nitrogen and hydrogen while using the cage as a cathode and a inner wall of the processing furnace as an anode (ion nitriding).
The nitriding method for obtaining the cage of the present invention (that is, a method of forming a nitride layer comprising xcex3xe2x80x2 phase, xcex5 phase or (xcex3xe2x80x2+xcex5) phase to the surface of a cage formed of an iron and steel material with the carbon content of 0.25% by weight or less) is not restricted to the methods (1) - (4) described above.
It is preferred that the cage of the present invention has a diffusion layer formed by dispersion of nitrogen in a matrix just bellow the nitride layer at a thickness of 50 xcexcm or more and 500 xcexcm or less, in which the vickers hardness (H1) is 160 or more at a position for the depth of 30 xcexcm from the boundary between the diffusion layer and the nitride layer, and the ratio (H1/H2) of the vickers hardness (H1) of the diffusion layer relative to the hardness (H2) of the core portion; is 1.30 or more. It is further preferred that the hardness (H1) of the diffusion layer is 223 or more by the vickers hardness and the ratio(H1/H2) is 2.50 or less.
Further, in view of the impact resistance, the hardness(H1) of the diffusion layer is preferably 300 or less and, further preferably, 280 or less by the vickers hardness. However, in an application use requiring not so high impact resistance since no large impact is applied, the hardness (H1) of the diffusion layer may exceed 300 in the vickers hardness and the upper limit for the hardness in this case is, for example, 350 in the vickers hardness.
When nitriding is applied to the iron and steel material comprising low carbon steel such as SPCC or SPDH (iron and steel material with carbon content of 0.25% by weight or less), a diffusion layer in which nitrogen is diffused in the matrix is formed just below the nitride layer formed on the surface (core side). The mechanical strength of cage is increased by defining the diffusion layer to the constitution described above.
Further, the pressed cage is work hardened by plastic work strain applied upon pressing. As described above, the work hardening can be kept by applying the nitriding in a temperature range not causing recrystallization in the low carbon steel (preferably at 400-500xc2x0 C., more preferably, 400-540xc2x0 C.). When the work hardening is kept, the diffusion layer of the constitution described above can be formed just below the nitride layer. The nitriding method capable of keeping the work hardening also after nitriding can include, for example, the methods (1)-(4) described above.
Also in the case of applying the nitriding at a temperature higher than the recrystallization temperature of steel, the diffusion layer of the constitution described above can be formed just bellow the nitride layer by (1) a method of increasing the cooling rate after nitriding (conducting oil cooling or water cooling), or (2) a method of applying a predetermined post treatment after cooling (air cooling or gradual cooling) by a reduced rate.
In a state where nitrogen in the diffusion layer reacts with iron as a matrix to precipitate barxe2x80x94shape or needlexe2x80x94shape nitrides, the strength of the matrix is lowered due to lowering of the nitrogen concentration in the matrix (ferrite) (refer to FIG. 6). Precipitation of nitrides can be suppressed by the method (1) and (2) described above.
The post treatment conducted in the method(2)can include a method of heating in atmospheric air or in an inert gas such as argon or nitrogen to a temperature of 300xc2x0 C. or higher and then applying oil cooling or water cooling. According to this method, since the cooling rate after nitriding is slow, and nitrides (Fe4N) precipitated in the diffusion layer are solid solubilized into the matrix in the heating step described above, the nitrogen concentration in the matrix is increased. According to the method (2), since the temperature for starting rapid cooling can be lowered compared with the method (1), it also has an effect capable of decreasing the deformation of the cage.
It is preferred that the cage of the present invention has an oxide layer comprising an Fe3O4 phase as a main ingredient and not containing an Fe2O3 phase to a thickness of 50 nm or more nearer to a further surface on the nitride layer.
The cage having an oxide layer comprising the Fe3O4 phase as a main ingredient and not containing the Fe2O3 phase on the surface has increased self-lubricity compared with the case of not having the oxide layer. Provision of the nitride layer under the oxide layer can provide the self-lubricity of the oxide layer effectively and can enhance the corrosion resistance. When the thickness of the oxide layer is less than 50 nm, no substantial effect can be obtained.